The proposal is to investigate the function of the NKG2D immunoreceptor in immune responses to tumor cells. NKG2D is a stimulatory receptor expressed by all natural killer (NK) cells, all activated cytotoxic T cells (CTL), all activated macrophages, and fractions of gamma/delta T cells and NK1.1+ T cells. NKG2D recognizes several related cell surface proteins (ligands), including Rael and H60. Expression of high levels of NKG2D ligands by target cells results in a strong attack by NK cells and activated macrophages, and an enhanced response of CTL to tumor-specific antigens. Most normal cells do not express these NKG2D ligands. Most tumor cell lines, in contrast, upregulate Rael and other NKG2D ligands. The hypothesis to be tested is that NKG2D and its ligands represent a system for tumor immunosurveillance. That is, it is proposed that NKG2D ligands are normally upregulated in developing tumor cells as a means to alert and activate the immune system, and that the resulting immune response serves to limit the development and/or growth of tumors in normal animals. This hypothesis will be tested using several approaches. In addition, preliminary experiments demonstrate that tumor cell lines transduced to express high levels of NKG2D ligands, when irradiated, function as potent therapeutic cancer vaccines that stimulate an effective immune response against established tumors of the same type (but not transduced). Therefore, studies will be undertaken to establish how effective this approach is and whether it can be effectively combined with other immunotherapies. Specific Aim 1: To determine the role of NKG2D isoforms associated with different adapter molecules in stimulating and costimulating NKG2D+ cells. Specific Aim 2: To generate and characterize gone targeted mice lacking NKG2D expression. Specific Aim 3: To assess the importance of NKG2D in surveillance of tumors in vivo. Specific Aim 4: To determine the efficacy of NKG2D stimulation as a cancer immunotherapy